I have dealt with these symptoms on and off over the years. They could never give me a difinitive answer but here's what I've found out:
Upon standing up when it gets dark around you - that's low blood pressue that has to do with your heart not pumping blood fast enough from your legs. I had that frequently as a young person when I was very physically fit and thin. Now it's rare but my young thin, fit daughter now deals with with. The only solution - anticipate it and stand up slowly.
The other dizzy stuff - I've noticed that this occurs more often at certain time of the month so I think it's affected by hormones. It also seems connected to sinus / inner ear issues.
It's also connected to the amount of sleep I'm getting. I'm getting to the peri-menopausal time of my life and there are some nights that I don't sleep well. During those days I tend to have more dizzy spells.
Keep properly hydrated - drink LOTS of water. That helps too.
There are a lot of nutritionist who say although we eat all these "good" foods we are still suffering from malnutrition as a result of not having proper enzymes needed to digest the foods. This could all be true but since the work-up is not covered by my helath plan and I didn't have hundreds of dollars to pay for it I never pursued it.
Finally my MD told me a couple of years ago that I have a very slight electrical issue with my heart. My mother got a pacemaker at about age 63 - which is used to "fix" electrical problems with the heart. My doc said my electrical issue may progress or may not but it was benign at the time. It's called a AV node block. My sister and my neice also have a similar issue. I'm not sure if that could be a part of it but I do intend to get it all looked at one day when I have a few minutes!
I would be curious to hear any follow up if you do get an answer from a doctor.